1. Field of the Invention
The present invention relates to a heterocyclic compound, a light-emitting element, a light-emitting device, an electronic device, and a lighting device. The invention further relates to an organic compound used in synthesis of the heterocyclic compound.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting material is interposed between a pair of electrodes. By applying voltage to this element, light emission from the light-emitting material can be obtained.
Since such a light-emitting element is of self-light-emitting type, it is considered that the light-emitting element has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not required, and so on and is therefore suitable as flat panel display elements. Besides, such a light-emitting element has advantages in that it can be formed to be thin and lightweight, and has quite fast response speed.
Furthermore, such a light-emitting element can be formed in a film form, and thus can provide planar light emission. Therefore, a large-area element can be easily formed. This is a feature which is difficult to be obtained by point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Accordingly, the light-emitting element is extremely effective for use as a surface light source applicable to illumination and the like.
Light-emitting elements utilizing electroluminescence can be broadly classified according to whether a light-emitting material is an organic compound or an inorganic compound. In the case of an organic EL element in which a layer containing an organic compound used as a light-emitting material is provided between a pair of electrodes, application of voltage to the light-emitting element causes injection of electrons from a cathode and holes from an anode into the layer containing the organic compound having a light-emitting property and thus current flows. The injected electrons and holes then lead the organic compound having a light-emitting property to its excited state, whereby light emission is obtained from the excited organic compound having a light-emitting property.
Note that excited states of the organic compound include a singlet excited state and a triplet excited state. Light emission from the singlet excited state (S*) is referred to as fluorescence, and light emission from the triplet excited state (T*) is referred to as phosphorescence. In addition, the statistical generation ratio in a light-emitting element is considered to be S*:T*=1:3.
With a compound that can convert energy of a singlet excited state into light emission (hereinafter, called fluorescent compound), only light emission from the singlet excited state (fluorescence) is observed and that from the triplet excited state (phosphorescence) is not observed, at room temperature. Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25%, on the basis of S*:T*=1:3.
In contrast, with a compound that can convert energy of a triplet excited state into light emission (hereinafter, called phosphorescent compound), light emission from the triplet excited state (phosphorescence) is observed. Further, since intersystem crossing (i.e., transition from a singlet excited state to a triplet excited state) easily occurs in a phosphorescent compound, the internal quantum efficiency can be theoretically increased to 100%. That is, higher emission efficiency can be achieved than using a fluorescent compound. From this reason, in order to achieve a high-efficiency light-emitting element, a light-emitting element using a phosphorescent compound has been actively developed recently.
When a light-emitting layer of a light-emitting element is formed using a phosphorescent compound described above, in order to suppress concentration quenching or quenching due to triplet-triplet annihilation of the phosphorescent compound, the light-emitting layer is often formed such that the phosphorescent compound is dispersed in a matrix of another compound. Here, the compound serving as the matrix is called host material, and the compound dispersed in the matrix, such as a phosphorescent compound, is called guest material.
When a phosphorescent compound is used as the guest material, a property required of the host material is that the host material have a greater triplet excitation energy (the difference in energy between the ground state and the triplet excited state) than that of the phosphorescent compound.
Furthermore, because the singlet excitation energy (the difference in energy between the ground state and the singlet excited state) is greater than the triplet excitation energy, a substance that has a high triplet excitation energy also has a high singlet excitation energy. Therefore, the above substance that has high triplet excitation energy is also effective in a light-emitting element using a fluorescent compound as a light-emitting material (guest material).
Studies have been conducted on compounds having dibenzo[f,h]quinoxaline rings, which are examples of the host material used when a phosphorescent compound is a guest material (e.g., see Patent Documents 1 and 2).